BEIJING — China has labeled a recent mission by nuclear-capable U.S. B-52 bombers over the disputed South China Sea as "provocative."

Defense Ministry spokesman Ren Guoqiang said at a monthly briefing Thursday that China would respond with all measures considered necessary to safeguard its rights and interests.

Two B-52s flew over the strategic waterway claimed by China earlier this week in what the Pentagon called a routine mission.

Asked Wednesday about the flights, U.S. Defense Secretary Jim Mattis responded that, "there's nothing out of the ordinary about it."

China has sought to strengthen its claim to the South China Sea by building seven islands on reefs and equipping them with military facilities such as airstrips, radar domes and missile systems. Five other governments claim territory in the area.

Before President Rodrigo Duterte left Thursday for his one-on-one with President Xi Jinping in Beijing, he promised to “invoke” the 2016 ruling of the Permanent Court of Arbitration at The Hague that resolved some maritime disputes between the two neighbors.

Eight warships, four aircraft and more than a thousand personnel from the US and ten Southeast Asian countries will join maritime drills kicking off Monday, as part of a joint exercise extending into the flashpoint South China Sea.

China has rejected as “unwelcome” the call of the United Kingdom, France and Germany on the South China Sea claimants to respect the arbitration ruling of 2016 and the rules-based framework laid out in the United Nations Convention on the Law of the Sea (UNCLOS).